HI all, just thinking a bit about that aluminum jig. Why not place to longer pieces of aluminum L bracket to make the rails? That would work just fine and be very easy. You want the rail parallel and a fairly tight fit on the brackets on the ends of the jig. You might find a rectangular extrusion that's a little taller, You probably don't want this thing longer thanb 60 inches, which is just right for BB ply, so it stays flat. Voila, there's your whole Woodpecker gadget for about $60 bucks.
I'd make the platform first, then add the rails. Use two pieces of scrap to space the rails to make sure they're parallel, and start them from the top of the ply so they're even. Then make the jig and fit it to the rails. Fit the end and side pieces and fix them in place with clamps and drill holes to secure it. I'd use rivits to assemble it, at least three per end. I'd also use an engineer's square to make sure the jig was square as possible, and a short chunk of scrap ripped to a hair more than the diameter of the plunge base. If you really wanted to complicate your life, you could make a square base for the router, that that would be completely unnecessary.